The present invention relates to a method of performing chemical mechanical polishing in a planarization process during the manufacture of a semiconductor wafer or a semiconductor integrated circuit formed on the semiconductor wafer.
To planarize a surface of a semiconductor wafer with stepped portions formed thereon in the process of manufacturing the wafer or to planarize a circuit pattern with rugged surface topography produced in the process of manufacturing a semiconductor integrated circuit, there has recently been used chemical mechanical polishing (hereinafter referred to as CMP). Referring to FIG. 7, a conventional apparatus for polishing a semiconductor wafer by using CMP will be described. FIG. 7 shows a structure of the conventional apparatus for polishing a semiconductor wafer, in which a slurry supply tank 1 reserves therein a polishing slurry 2A which is a coloidal-suspensiontype polishing slurry containing abrasive particles in the solution. The polishing slurry 2A reserved in the slurry supply tank 1 is conveyed under pressure through a slurry supply pipe 4 by a slurry feed pump 3 and supplied from a slurry outlet 5 to a surface of a polishing cloth 7 affixed to a flat and smooth surface of a polishing platen 6 such that the surface of the polishing cloth 7 is coated with a polishing slurry 2B supplied thereto. A semiconductor wafer 9 held by a wafer carrier 8 has a surface pressed against the surface of the polishing cloth 7 and performs relative movement, such as rotation or translation, between the wafer carrier 8 and the polishing platen 6, whereby the surface of the semiconductor wafer 9 is polished. A discharged slurry containing the polishing slurry discharged from the surface of the polishing cloth during polishing is received by a discharged slurry receptacle 10 and drained through a discharged slurry pipe 11.
However, the conventional apparatus with the structure described above has encountered the problem of agglomeration of a plurality of abrasive particles in the polishing slurry. When the pH is held constant, the surfaces of the abrasive particles in the polishing slurry are normally charged to have the same polarity, so that the polishing particles repel one another by electrostatic repulsion to be uniformly dispersed and floated in the polishing slurry. However, since the polishing slurry is a coloidal solution containing the abrasive particles, it forms a viscous flow when conveyed under pressure from the slurry supply tank 1 through the slurry supply pipe 4. This causes a friction between the polishing slurry 2A and the inner wall of the slurry supply pipe 4 and a friction within the polishing slurry 2A so that the charged state on the surfaces of the abrasive particles is changed thereby. On some occasions, the abrasive particles may be attracted to each other by an electrostatic force and agglomerated, resulting in an apparently single particle with a large diameter formed of the agglomerated abrasive particles.
Likewise, the charged state on the surfaces of the abrasive particles is also changed by the friction between the polishing slurry 2B and the polishing cloth 7 during polishing, which may cause the agglomeration of the abrasive particles and produce an apparently single particle with a large diameter formed of the agglomerated abrasive particles.
Furthermore, an abrupt pH change during the water-washing of the polishing cloth 7 also induces a change in the charged state on the surfaces of the abrasive particles, which may cause the agglomeration of the abrasive particles and produce an apparently single particle with a large diameter formed of the agglomerated abrasive particles.
A description will be given to the phenomenon of agglomeration of the abrasive particles with reference to FIGS. 7 to 10. FIG. 8 shows the distribution of the diameters of the abrasive particles in the polishing slurry 2A charged into the slurry supply tank 1 shown in FIG. 7, i.e., the initial distribution of the diameters of the particles. It is assumed that the abrasive particles contained in the polishing slurry 2A are composed of commercially available coloidal silica. FIG. 9 shows the distribution of the diameters of the particles in the polishing slurry 2B collected from the slurry outlet 5 shown in FIG. 7. FIG. 10 shows the distribution of the diameters of the particles remaining on the surface of the polishing cloth 7 after a plurality of silicon wafers with respective oxide films were polished by the apparatus f or polishing a semiconductor wafer shown in FIG. 7. From the comparison between the distributions of the diameters of the particles shown in FIGS. 8 to 10, the following findings were achieved. Specifically, it was found from the comparison between FIGS. 8 and 9 that the polishing slurry 2B supplied from the slurry supply tank 1 through the slurry supply pipe 4 contained particles with diameters larger than the diameters of the particles initially contained in the polishing slurry. This indicates that the abrasive particles started to agglomerate while the polishing slurry flew through the slurry supply pipe 4, not that the individual particles were increased in size. Therefore, the particle having a large diameter of 3.0 to 10 .mu.m shown in FIG. 9 is a single particle formed of a plurality of agglomerated abrasive particles. It was also found from the comparison between FIGS. 9 and 10 that the particles remaining on the surface of the polishing cloth 7 include particles with much larger diameters than the particles contained in the polishing slurry 2B supplied to the surface of the polishing cloth 7 and that the agglomerated abrasive particles were increased in number. This indicates that the agglomeration of the abrasive particles newly occurred during polishing and proceeded in conjunction the agglomeration of the abrasive particles occurred prior to polishing. The abrasive particles agglomerated during the CMP process not only renders polishing properties including a polishing rate unstable but also causes a scratch on the surface of the semiconductor wafer 9 as a workpiece to be polished. Since the scratch may lead to a pattern defect during the process of forming a circuit pattern after the CMP process, the yield of the semiconductor integrated circuit as well as the yield of the semiconductor wafer are reduced.